1. Field of the Invention
The present invention generally relates to a pressure-adjustable container, and more particularly to such containers that are typically made of polyester and are capable of being filled with hot liquid. It also relates to an improved sidewall construction for such containers.
2. Related Art
The use of blow molded plastic containers for packaging “hot fill” substances is well known. However, a container that is used for hot fill applications is subject to additional mechanical stresses on the container that result in the container being more likely to fail during storage or handling. For example, it has been found that the thin sidewalls of the container deform or collapse as the container is being filled with hot fluids. In addition, the rigidity of the container decreases immediately after the hot fill liquid is introduced into the container. As the liquid cools, the liquid shrinks in volume which, in turn, produces a negative pressure or vacuum in the container. The container must be able to withstand such changes in pressure without failure.
Hot fill containers typically comprise substantially rectangular vacuum panels that are designed to collapse inwardly after the container has been filled with hot liquid. However, the inward flexing of the panels caused by the hot fill vacuum creates high stress points at the top and bottom edges of the panels, especially at the upper and lower comers of the panels. These stress points weaken the portions of the sidewall near the edges of the panels, allowing the sidewall to collapse inwardly during handling of the container or when containers are stacked together. See, for example, U.S. Pat. No. 5,337,909.
“Hot fill” applications impose significant and complex mechanical stress on a container structure due to thermal stress, hydraulic pressure upon filling and immediately after capping, and vacuum pressure as the fluid cools.
Thermal stress is applied to the walls of the container upon introduction of hot fluid. The hot fluid will cause the container walls to soften and then shrink unevenly, causing distortion of the container. The polyester typically used to form the container must therefore be heat-treated to induce molecular changes resulting in a container that exhibits thermal stability. Pressure and stress are acted upon the sidewalls of a heat resistant container during the filling process, and for a significant period of time thereafter. When the container is filled with hot liquid and sealed, there is an initial hydraulic pressure and an increased internal pressure is placed upon containers. As the liquid, and the air headspace under the cap, subsequently cool, thermal contraction results in partial evacuation of the container. The vacuum created by this cooling tends to mechanically deform the container walls.
Generally speaking, containers incorporating a plurality of longitudinal flat surfaces accommodate vacuum force more readily. Agrawal et al, U.S Pat. No. 4,497,855 discloses a container with a plurality of recessed collapse panels, separated by land areas, which allows uniformly inward deformation under vacuum force. The vacuum effects are controlled without adversely affecting the appearance of the container. The panels are drawn inwardly to vent the internal vacuum and so prevent excess force being applied to the container structure, which would otherwise deform the inflexible post or land area structures. The amount of “flex” available in each panel is limited, however, and as the limit is approached there is an increased amount of force that is transferred to the side walls.
The flexure is most commonly addressed with vacuum flex panels positioned under a label below the dome of the container. One example of containers having such vacuum flex panels is disclosed in U.S. Pat. No. 5,141,120 (Brown et al.) and U.S. Pat. No. 5,141,121 (Brown et al.), each of which is incorporated by reference. In such patents, pinch grip indentations function as the vacuum flex panels. Another example of containers having such vacuum flex panels is disclosed in U.S. Pat. No. 5,392,937 (Prevot et al.) and U.S. Des. Pat. No. 344,457 (Prevot et al.), both of which are assigned to the assignee of the present invention and hereby incorporated by reference. In those containers, a grip structure moves with the vacuum flex panel in response to vacuum induced inside the container in response to hot filling, capping and cooling of the container contents. Still another example of containers having such vacuum flex panels is disclosed in International Publication No. WO 00/50309 (Melrose), which is incorporated herein by reference. With that container, a controlled deflection vacuum flex panel inverts and flexes under pressure to avoid deformation and permanent buckling of the container. It includes an initiator portion, which has a lesser projection than the remainder of the flex panel and initiates deflection of the flex panel.
External forces are applied to sealed containers as they are packed and shipped. Filled containers are packed in bulk in cardboard boxes, or plastic wrap, or both. A bottom row of packed, filled containers may support several upper tiers of filled containers, and potentially, several upper boxes of filled containers. Therefore, it is important that the container have a top loading capability, which is sufficient to prevent distortion from the intended container shape. Dome region ovalization is a common distortion associated with hot-fillable, blow-molded plastic containers. The dome is the upper portion of the container adjacent the finish. Some dome configurations are designed to have a horizontal cross-section which is circular in shape. The forces resulting form hot-filling and top loading can change the intended horizontal cross-sectional shape, for example, from circular to oval.
Examples of hot-fillable, blow-molded plastic containers that can withstand the above referenced forces and can maintain their as-designed aesthetic appearance are the containers disclosed in U.S. Pat. Nos. Des. 366,416, 366,417, and 366,831 all assigned to the assignee of the present application and incorporated herein by reference. The referenced design patents illustrate in phantom lines a “bell-shape” dome located between a finish and a label mounting area. The diameter of the horizontal cross-section through a bell-shaped dome increases as the dome extends downwardly from the finish. The dome diameter then decreases to an inwardly extending peripheral waist, and downwardly from the waist, the dome diameter increases before connecting with the label mounting area of the container. The bell-shape of the dome provides an aesthetic appearance as initially blow-molded, and it provides a degree of reinforcement against distortion of the dome, particularly ovalization types of distortion.
To minimize the effect of force being transferred to the side walls, much prior art has focused on providing stiffened regions to the container, including the panels, to prevent the structure yielding to the vacuum force. The provision of horizontal or vertical annular sections, or ‘ribs’, throughout a container has become common practice in container construction, and is not only restricted to hot-fill containers. Such annular sections will strengthen the part they are deployed upon. U.S. Pat. No. 4,372,455 (Cochran) discloses annular rib strengthening in a longitudinal direction, placed in the areas between the flat surfaces that are subjected to inwardly deforming hydrostatic forces under vacuum force. U.S. Pat. No. 4,805,788 (Ota et al.) discloses longitudinally extending ribs alongside the panels to add stiffening to the container. Ota also discloses the strengthening effect of providing a larger step in the sides of the land areas. This provides greater dimension and strength to the rib areas between the panels.
U.S. Pat. No. 5,178,290 (Ota et al.) discloses indentations to strengthen the panel areas themselves. U.S. Pat. No. 5,238,129 (Ota et al.) discloses further annular rib strengthening, this time horizontally directed in strips above and below, and outside, the hot-fill panel section of the bottle. In addition to the need for strengthening a container against both thermal and vacuum stress, there is a need to allow for an initial hydraulic pressure and increased internal pressure that is placed upon a container when hot liquid is introduced followed by capping. This causes stress to be placed on the container side wall. There is a forced outward movement of the heat panels, which can result in a barreling of the container.
Thus, U.S. Pat. No. 4,877,141 (Hayashi et al.) discloses a panel configuration that accommodates an initial, and natural, outward flexing caused by internal hydraulic pressure and temperature, followed by inward flexing caused by the vacuum formation during cooling. Importantly, the panel is kept relatively flat in profile, but with a central portion displaced slightly to add strength to the panel but without preventing its radial movement in and out. With the panel being generally flat, however, the amount of movement is limited in both directions. By necessity, panel ribs are not included for extra resilience, as this would prohibit outward and inward return movement of the panel as a whole.
U.S. Pat. No. 5,908,128 (Krishnakumar et al.) discloses another flexible panel that is intended to be reactive to hydraulic pressure and temperature forces that occur after filling. Relatively standard ‘hot-fill’ style container geometry is disclosed for a “pasteurizable” container. It is claimed that the pasteurization process does not require the container to be heat-set prior to filling, because the liquid is introduced cold and is heated after capping. Concave panels are used to compensate for the pressure differentials. To provide for flexibility in both radial outward movement followed by radial inward movement however, the panels are kept to a shallow inward-bow to accommodate a response to the changing internal pressure and temperatures of the pasteurization process.
The increase in temperature after capping, which is sustained for some time, softens the plastic material and therefore allows the inwardly curved panels to flex more easily under the induced force. It is disclosed that too much curvature would prevent this, however. Permanent deformation of the panels when forced into an opposite bow is avoided by the shallow setting of the bow, and also by the softening of the material under heat. The amount of force transmitted to the walls of the container is therefore once again determined by the amount of flex available in the panels, just as it is in a standard hot-fill bottle. The amount of flex is limited, however, due to the need to keep a shallow curvature on the radial profile of the panels. Accordingly, the bottle is strengthened in many standard ways.
U.S. Pat. No. 5,303,834 (Krishnakumar et al.) discloses still further “flexible” panels that can be moved from a convex position to a concave position, in providing for a “squeezable” container. Vacuum pressure alone cannot invert the panels, but they can be manually forced into inversion. The panels automatically “bounce” back to their original shape upon release of squeeze pressure, as a significant amount of force is required to keep them in an inverted position, and this must be maintained manually. Permanent deformation of the panel, caused by the initial convex presentation, is avoided through the use of multiple longitudinal flex points.
U.S. Pat. No. 5,971,184 (Krishnakumar et al.) discloses still further “flexible” panels that claim to be movable from a convex first position to a concave second position in providing for a grip-bottle comprising two large, flattened sides. Each panel incorporates an indented “invertible” central portion. Containers such as this, whereby there are two large and flat opposing sides, differ in vacuum pressure stability from hot-fill containers that are intended to maintain a generally cylindrical shape under vacuum draw. The enlarged panel side wails are subject to increased suction and are drawn into concavity more so than if each panel were smaller in size, as occurs in a ‘standard’ configuration comprising six panels on a substantially cylindrical container. Thus, such a container structure increases the amount of force supplied to each of the two panels, thereby increasing the amount of flex force available. Even so, the convex portion of the panels must still be kept relatively flat, however, or the vacuum force cannot draw the panels into the required concavity.
The need to keep a shallow bow to allow flex to occur was previously described by Krishnakumar et al. in both U.S. Pat. No. 5,303,834 and U.S. Pat. No. 5,908,128. This, in turn, limits the amount of vacuum force that is vented before strain, is placed on the container walls. Further, it is generally considered impossible for a shape that is convex in both the longitudinal and horizontal planes to successfully invert, anyhow, unless it is of very shallow convexity. Still further, the panels cannot then return back to their original convex position again upon release of vacuum pressure when the cap is removed if there is any meaningful amount of convexity in the panels. At best, a panel will be subject to being “force-flipped” and will lock into a new inverted position. The panel is then unable to reverse in direction as there is no longer the influence of heat from the liquid to soften the material and there is insufficient force available from the ambient pressure. Additionally, there is no longer assistance from the memory force that was available in the plastic prior to being flipped into a concave position.
U.S. Pat. No. 5,908,128 (Krishnakumar et al.) previously disclose the provision of longitudinal ribs to prevent such permanent deformation occurring when the panel arcs are flexed from a convex position to one of concavity. This same observation regarding permanent deformation was also disclosed in U.S. Pat. No. 5,303,834 (Krishnakumar et al.) U.S. Pat. No. 4,877,141 (Hayashi et al.) also disclosed the necessity of keeping panels relatively flat if they were to be flexed against their natural curve.
Thus, previous hot-fill containers usually include horizontal or vertical annular sections or ‘ribs’, to provide stiffness and increase structural support. These additional support structures create crevices and recesses in the interior of the container. When the container stores a viscous substance, such as jelly, jam, preserves, or heavy syrup, the viscous substance may become trapped in these crevices and recesses. Accordingly, a consumer may have difficulty accessing and removing a viscous substance from the container.
The present invention in contrast, allows for increased flexing of the vacuum panel side walls so that the pressure on the containers may be more readily accommodated, while eliminating as much geometry inside of the container as possible to facilitate product removal.